LONDON, July 11 (Reuters) - British Foreign Secretary Liz Truss entered the race to replace Boris Johnson as prime minister on Monday, taking the number of candidates in an increasingly bitter and unpredictable contest to 11.

Truss, who has held ministerial jobs in a number of government departments including trade, justice and the treasury, said she would slash taxes and maintain a tough line against Russian President Vladimir Putin.

She is seeking to replace Johnson who was forced out on Thursday after his government imploded over a series of scandals.

The contest is for the leadership of the ruling Conservative Party, with the winner then becoming prime minister. The aim is to find a successor by September.

"I will lead a government committed to core Conservative principles: low taxes, a firm grip on spending, driving growth in the economy, and giving people the opportunity to achieve anything they want to achieve," she said in a campaign video.

The race followed one of the most remarkable periods in modern British political history, when more than 50 government ministers quit, denouncing Johnson's character, integrity and inability to tell the truth.

With many lawmakers unhappy with the disgraced Johnson remaining in office until a successor is found, the party is likely to accelerate the election process. It could insist that candidates have the backing of around 30 lawmakers to enter the process, before voting begins this week to whittle the number down to two.

The issue of tax cuts was fast becoming the central battle in the race with nearly all of the candidates promising to cut business or personal taxes.

Setting out her pitch to the Conservative Party's 200,000 members who will decide the outcome of the contest, Truss said she would reverse the recent rise in National Insurance contributions and signalled a cut to corporation tax.

Fellow contenders Jeremy Hunt and Sajid Javid have both pledged to cut corporation tax, while the former defence minister Penny Mordaunt has promised to cut fuel duty.

Former finance minister Rishi Sunak is the early front runner, but he is the only candidate who has played down the prospect of imminent tax cuts, saying the adoption of "comforting fairy tales" would leave future generations worse off.

"Someone has to grip this moment," he said in his launch video.

This has prompted his rivals to attack his economic record after the tax burden rose to the highest level since the 1950s. One lawmaker confirmed that a dossier criticising Sunak's record had been circulating on lawmaker WhatsApp groups.

Nadhim Zahawi, appointed finance minister in the turmoil of last week, said he was also being targeted by rivals after media reports raised questions about the former businessman's personal finances and tax record.

"I was clearly being smeared," he told Sky News.

Whoever wins the leadership race will be faced with a daunting in-tray.

Britain's economy is facing rocketing inflation, high debt, and low growth, with people coping with the tightest squeeze on their finances in decades, all set against a backdrop of an energy crunch exacerbated by the war in Ukraine which has sent fuel prices soaring.

On the issue of immigration, all the main leadership candidates have pledged to keep the government's policy of sending asylum seekers to Rwanda, showing how the party has moved to the right of the political spectrum in recent years.

Other candidates include the attorney general, Suella Braverman, the chair of parliament's foreign affairs committee Tom Tugendhat, and the transport secretary Grant Shapps.